The effects of ion-implanted layers together with deposited surface films on resurfed LDMOS transistors is studied. The specific on-resistance of lateral DMOS transistors is calculated and compared for two types of device, the `resurfed´ structure and a surface implanted SIPOS covered structure. Device parameters are first chosen so as to give 600 V breakdown for each type. It is shown that an LDMOS device with a surface implanted layer and SIPOS overlay gives superior performance with respect both to on resistance and insensitivity of the breakdown voltage to fluctuations in substrate doping density.